1. Field of Invention
The present invention relates to an operational transconductance amplifier (OTA), and particularly to an operational transconductance amplifier (OTA) capable of eliminating the static current component contained in the output current thereof.
2. Description of the Related Art
A drawback of a conventional operational transconductance amplifier (OTA) to perform a unilateral conversion of signals (voltage-to-current converting) is that a static current component is always companied with the output current therefrom, which though is able to reach a desired converting speed, makes the following circuit difficult to precisely process the AC current component containing useful information from the output current. To overcome the drawback, a conventional OTA shown in FIG. 1 is required, which is capable of eliminating the unwanted static current component in the output current. Referring to FIG. 1, an OTA 100 includes a differential unit 121, a differential unit 122 and a voltage-to-current converting unit 130. Wherein, the differential unit 121 includes transistors T3 and T5, while the differential unit 122 includes transistors T4 and T6.
Normally, the OTA 100 is used in a high-speed circuit. Wherein, the differential unit 121 is biased by a current mirror formed by a reference current source 110 and transistors T0 and T1 and is biased by a current source 111. The differential unit 122 is biased by a current mirror formed by a reference current source 110 and transistors T0 and T2 and is biased by a current source 112. In the current mirror, a static current component IS0 provided by the reference current source 110 would flow through the transistor T0 and makes the transistors T1 and T2 produce a current proportional to the static current IS0, respectively. For example, the transistor T1 would produce a static current I1, where I1=m×IS0.
Generally, the transistors T1 and T2 are designed to have the same aspect ratio (i.e. W/L ratio of a transistor where W and L denote channel width and channel length of a transistor). In the same way, the transistors T3 and T4 have the same aspect ratio. Thus, the static currents passing through the transistors T1˜T4 are the same. The differential units 122 and 121 receive a voltage signal Vin and a voltage signal Vref, respectively, and a voltage-to-current converting unit 130 converts the voltage difference of the voltage signals Vin and Vref into a current signal iR. In an embodiment, the voltage-to-current converting unit 130 is realized by a resistor with a resistance R, hence, iR=(Vin−Vref)/R. At the point, if there is a difference between the voltage signals Vin and Vref, the current signal iR and a static current difference (IS1−I1) would forcedly pass through the transistor T5, so that the current signal i5=(IS1−I1)+iR, where (IS1−I1) is the above-mentioned static current component and iR is the above-mentioned AC current component containing information.
In order to eliminate the static current component (IS1−I1) in the output current Iout, two transistors T5 and T7 are employed to transfer the current signal i5 on the transistor T5, via an output node N1, to the transistor T7 for producing a current signal i7, wherein the aspect ratio of the transistor T5 is proportional to the aspect ratio of the transistor T7 in a factor. If the aspect ratio of the transistor T7 is n times as large as the aspect ratio of the transistor T5, the current signal i7 would be i7=n×i5=n×[(IS1−I1)+iR]. Thus, the current provided by the current source 113 must, by design, be IS3=n×(IS1−I1) for eliminating the static current (IS1−I1) in the output current Iout.
Since the voltage at the output node N1 is different from the voltage at the drain terminal of the transistor T0 (note that the drain terminal connects the gate terminal), the circuit is very sensitive to a channel length modulation of transistors, which leads to a drift of the static current I1 provided by the transistor T1 resulting in a complete unmatching between the static current I1 and the current IS0 passing through the transistor T0, so that the current source 113 originally designed for eliminating the static current (IS1−I1) has a much reduced compensation effect.